This invention relates to a collection of loose composite plateletlike particles comprising a core and at least one coating layer consisting essentially of a compound having from 60 to 95% by weight of carbon and from 5 to 25% by weight of nitrogen, the balance to 100% being selected from elements of the group consisting of hydrogen, oxygen and sulfur, as well as to processes for the manufacture thereof, to polymer compositions containing it, and to the use thereof as effect pigments.
Effect (or luster) pigments are reflective flat particles that show at least partly specular reflection of the incident light. In a surface painted with effect pigments, for example, the effect pigment particles in the paint usually orient themselves substantially parallel to the surface, so that the colored paint surface when illuminated by a fixed white light source shows a luster effect and may appear in different colors according to the angle at which it is viewed and the nature of the effect pigment. A high-quality coloured effect pigment should impart highly saturated colors to the medium in which it is incorporated at all viewing angles. An optically variable pigment should also have a large difference in color between different viewing angles (high goniochromaticity).
The visual difference between two colors is best reflected by the xcex94E* value in the L*a*b* color system (CIE-LAB 1986). Different types of effect pigments are able to impart effects to varying degrees; for example, simple metal particles, for example aluminium flakes, mainly produce differences in brightness (high xcex94L*), which in combination with transparent colored pigments leads to the so-called metallic flop effect.
In effect pigments, color is mainly produced by interference of light. Such pigments are particles that have been coated with a thin layer of a colorless or colored substance; the color effect depends on the thickness of the coating layer and may manifest itself both in the brightness (L*) and in the hue (H*). The goniochromaticity arises because the optical path length of the reflected beam is different at different angles to the surface.
Interference pigments can be prepared from any known plate-like particles, for example from plate-like organic or inorganic colored pigments, such as xcex2-copper phthalocyanine, 3,4,9,10-perylenetetracarboxylic acid diimides, fluororubins or xcex1-Fe2O3, from metal flakes, such as aluminium, copper or bronze flakes, or from silicatic particles.
However, the demands made of pigments are constantly increasing, so that the conventional effect pigments are unable fully to meet today""s high expectations, particularly in high-quality applications such as automotive lacquers. For example, many effect pigments which would be desirable from the point of view of hue often exhibit inadequate light or weather stability, and many interference pigments are lacking chroma (C*, saturation) and opacity.
In many cases, too, the mechanical strength of the effect pigments is not satisfactory. For example upon dispersion into an ink or paint composition, the coatings may break or peel off, leading to insatisfactory coloristics. This happens particularly with flat, smooth coatings which are desirable for coloristic reasons. Another problem is that it is very difficult to make relatively thick coatings without forming agglomerates, thus impairing the optical properties.
The preparation of fine black pigments through oxydative pyrolysis at 200-350xc2x0 C. of acrylonitrile-based polymer particles treated with an adhering aminosiloxane is disclosed in JP-63/142066-A. This process leads to an uniformly shaped black powder, which contains only traces of nitrogen and does not produce any luster effect or goniochromaticity.
DD 238 994 discloses organophilic colored fillers consisting of small clay or kaolin particles (Øxe2x89xa62 xcexcm) embedded in a matrix based on conducting polymers, which are obtained by calcination of a ceramic mass of clay and a polymer such as acrylonitrile at a temperature between 150xc2x0 C. and 50xc2x0 C. below the clay""s decomposition point. Yet, these composite fillers are of brown to black color, without any luster effect or goniochromaticity, and their components are not arranged regularly.
DD 238 993 discloses organophilic colored fillers consisting of small clay or kaolin particles (Ø less than 2 xcexcm) embedded in a matrix containing amorphous carbon, which are obtained by calcination of a ceramic mass of clay and a polymer such as acrylonitrile at a temperature above the clay""s decomposition point. Additional components such as mica may be contained in amounts up to 20% by weight. Yet, these composite fillers are of brown to black color, without any luster effect or goniochromaticity, and their components are not arranged regularly.
U.S. Pat. No. 5,322,561 relates to conductive flaky pigments, the conductive coating of which consists of a metal oxide pigment layer doped with additional metal oxide particles and containing interdispersed carbon black particles. The color is however black to pale and silvery grey, with quite a low chroma.
U.S. Pat. No. 3,087,827 discloses the deposition of carbon onto a TiO2 layer from hydrocarbons, fatty acids, fats or soaps at 700-1000xc2x0 C. The carbon fills into the minute spaces between the TiO2 particles, even when deposited at the end of the process. Total absence of oxygen is required in order to avoid undesirable soot or particulate carbon formation. Moreover, the products are insatisfactory light stable as is known from U.S. Pat. No. 5,501,731.
U.S. Pat. No. 5,271,771 discloses carbon-containing effect pigments which are obtained through simultaneous deposition of carbon and a metal on a plate-like substrate, and subsequent redox reaction between the metal oxide in the pigment""s undercoat and the metal in the pigment""s topcoat, together with precipitation of carbon, at high temperatures under reducing conditions. It is however not possible to deposit the carbon-containing layer without altering the system""s optical properties.
Dark effect pigments are known from DE-OS 195 02 231, which are coated with soot embedded in or overlaid with titanium oxide. They are obtained by coating a plateletlike core mechanically with soot particles, precipitating thereon titanium hydroxide and a metallic reducing agent, and pyrolizing the obtained composite at about 500-1000xc2x0 C. under inert conditions. The chroma is somewhat improved but at the detriment of the lightness which is much too low.
U.S. Pat. No. 4,076,551 discloses pigments coated with a metal hydroxide or bismuth oxychloride layer and carbon black particles incorporated therein. Example 3 discloses a blue mica/TiO2 interference pigment coated with 3% of carbon black and 0.73% Al2O3, which exhibits a strong dark blue powder color with a lively blue shimmer and may be heated to 300xc2x0 C. for 40 minutes without any gloss or color change. However, the amount of carbon which can be fixed is limited and depends on the pigment""s available surface area. For mica flakes, it does not exceed about 15 mg/m2, the carbon in excess remaining in suspension and affecting the luster. In addition, it is very difficult to disperse the carbon black in aqueous media, and the coating is irregular, so that the color and the goniochromaticity do not meet today""s requirements to a satisfactory extent.
U.S. Pat. No. 5,501,731 claims that some of above lacks may be solved by coating plateletlike silicatic substrates with carbon-containing metal compounds (such as CrIIIacac3) and compounds of the formula [(CH2O)1-6]x (such as sugars or starch), and then decomposing the carbon-containing compounds on the surface of the substrate particles under oxygen-excluding conditions. Very smooth coatings can allegedly be obtained when the decomposition takes place from the gas phase. However, this process leads to coatings containing high amounts of a metalxe2x80x94the ratio Cr/C is 0.92 in example 1 and 1.50 in example 2. Consequently, it is only suitable for very thin layers, generally 1-20 nm, preferably 1-10 nm. Furthermore, a substantial amount of the metal is detached from the coating upon thermal decomposition, leading to a highly undesirable contamination with metallic particles which affect the coloristic properties and can be abrasive or develop an undesired catalytic activity when the pigment is incorporated into a high molecular weight organic material.
U.S. Pat. No. 5,364,467 and U.S. Pat. No. 5,662,738 finally disclose luster pigments based on plateletlike metallic substrates comprising a first layer of metal oxide, a second, nonselectively absorbing layer of carbon, metal or metal oxide, and optionally a third layer of metal oxide. There is however no example wherein the second layer is carbon. Notwithstanding the statement that a carbon layer may be made by thermal decomposition of a compound containing at least 1 oxygen for every 2 carbon atoms (such as PVA, sorbitol or sugars), this method does not enable to make regularly coated, isolated particles. Instead, very irregularly coated particles are obtained together with agglomerates which consist of several platelets linked together at different dihedric angles by a bridging carbonaceous mass. Consequently, the coloristic properties of these luster pigments are still not satisfactory.
The instant invention""s object is to provide effect pigments that meet today""s requirements to an especially high degree even in high-quality applications. The effect pigments according to the invention possess superior optical properties, such as high reflectivity, brilliance, luster and opacity. Those of the instant effect pigments which are coloured display a high chroma coupled with interesting flop effects, for example goniochromaticity. Their outstanding light stability and chemical and mechanical properties render them particularly suitable for use in all customary kind of substrates, including water-based coating systems, wherein there is surprisingly no need for an additional stabilizing treatment even in the case of metallic cores.
The invention relates to a collection of composite plateletlike particles comprising a core and at least one coating layer consisting essentially of a compound having from 60 to 95% by weight of carbon and from 5 to 25% by weight of nitrogen, the balance to 100% being selected from elements of the group consisting of hydrogen, oxygen and sulfur.
Said coating is hereafter also referred to as a nitrogen doped carbon coating.
The compound""s carbon content is preferably from 70 to 90% by weight. The hydrogen content is preferably from 0.5 to 5% by weight. The nitrogen content is preferably from 13 to 22% by weight. The sulfur content is preferably below 1% by weight, most preferably nil. Preferably, there is a vast majority of loose particles, wherein a single core is surrounded by the instant nitrogen doped carbon coating. The number of loose particles is very preferably at least 80%, most preferably at least 95%, of the total number of loose and agglomerated particles. The nitrogen doped carbon coating around a core most preferably consists essentially of planar macromolecules arranged parallel to each other. Each core is preferably surrounded by one inventive coating.
Suitable core substrates for the luster pigments of the invention are transparent, partially reflectant or reflectant. Examples thereof are flat metallic or silicatic particles, graphite, Fe2O3, MoS2, talc or glass flakes, and plateletlike crystals of xcex2-phthalocyanine, fluororubine, red perylenes or diketopyrrolopyrroles. Silicatic particles are preferred, in particular light-colored or white micas, for example sericite, kaolinite, muscovite, biotite, phlogopite or related vermiculite, or any synthetic mica. Flakes of preferably wet-ground muscovite are particularly preferred, althought it is of course also possible to use other natural micas or artificial micas.
Another preferred embodiment is the use of flat metallic particles as the core. In contrast to previously known coatings, the instant coating can advantageously be made at temperatures below the melting point of the core metal, surprisingly enabling the preparation of perfectly shaped, coated metal flakes. Preferably, metal flakes are coated at temperatures below any phase change, as compared with their phase at room temperature.
Examples of suitable metallic particles are flakes of Ag, Al, Au, Cu, Cr, Fe, Ge, Mo, Ni, Si, Ti, or alloys thereof, such as brass or steel, preferably Al flakes. Depending on the material, a natural optically non-interfering oxide layer may form on the surface of metallic particle. Partially reflecting cores have preferably a reflectance of at least 35% of the light falling vertically on its surface in the range from 380 to 800 nm.
Surprising effects are obtained with all types of core materials. The cores may be colorless or colored and may consist of a single substance or of a combination of substances.
The instant pigments preferably also comprise an intermediate coating between the core and the nitrogen doped carbon coating, which intermediate coating may consist, for example, of one or more layers of Prussian blue, MgF2 or, especially, of a metal or mixed-metal oxide or oxide hydrate. Such pigments are well known to the person skilled in the art, for example from DE 32 07 936, EP 0 096 284 or U.S. Pat. No. 5,026,429. The intermediate layer has preferably a thickness of from 0.01 to 1 xcexcm.
On silicatic core particles, the intermediate layer consists preferably of a metal oxide, oxide hydrate or halide such as titanium, zirconium, tin, iron, chromium or zinc oxide, bismuth oxychloride or mixtures thereof, ontop which an optional protective layer may preferably also be applied to increase the stability, for example a layer of a metal oxide such as silicon or aluminium oxide. Of particular importance are micas, which are coated with highly refractive colorless metal oxides or oxide hydrates. Particularly preferred are intermediate coatings of zirconium dioxide or titanium dioxide; very particularly preferred is a coating of titanium dioxide. A very particular interest is given to micas having a dielectric coating layer of thickness from 0.03 to 0.3 xcexcm.
The intermediate coating layer may also consist of a pack of multiple layers, for example from 2 to 20 layers. The skilled artisan knows many types of multiple layers, which are all suitable, and which effects can be obtained therewith. If desired, a layer of a colorless metal oxide or oxide hydrate can for example be combined with a layer of a colored metal oxide or oxide hydrate. Or, layers having a high refractive index (xe2x89xa72.0) and layers having a low refractive index (xe2x89xa62.0) may be alternated. Multiple layer coatings are generally known as Fabry-Perot systems, many of which are known also in pigments technology, such as in U.S. Pat. No. 5,135,812.
On metallic flakes, the intermediate layer consists preferably of a metal oxide, oxide hydrate or halide such as titanium, zirconium, tin, iron, chromium or zinc oxide, bismuth oxychloride or mixtures thereof particularly preferred is a coating of silicium dioxide.
Particles coated with the above intermediate layers and their use as effect pigments are generally known per se, for example from DE 14 67 468, EP 0 045 851, DE 32 37 264, DE 36 17 430, EP 0 298 604, EP 0 388 932 and EP 0 402 943. Metal oxide-coated mica platelets are also commercially available unter the names Iriodin(copyright) (E. Merck, Darmstadt), Flonac(copyright) (Kemira Oy, Finland), Mearlin(copyright) (Mearl Corporation, New York/USA) and Infinite Color(copyright) (Shisheido, Japan). Coated metal flakes are also commercially available unter the names Chroma Flair(copyright) (Flex Products, Inc, Santa Rosa, Calif./USA) and Paliochrom(copyright) (BASF, Germany).
The size of the core particles is not critical per se and can be adapted to the particular use. Generally, the particles have a length from about 1 to 200 xcexcm, in particular from about 5 to 100 xcexcm, and thicknesses from about 0.05 to 5 xcexcm, preferably from 0.1 to 2 xcexcm, in particular about 0.5 xcexcm. Particles having a plateletlike shape are understood to be such having two essentially flat and parallel surfaces, with an aspect ratio length to thickness of from about 2:1 to about 1000:1, and a length to width ratio of from 3:1 to 1:1.
The nitrogen doped carbon coating may for example be prepared by any method known in the art, and then adsorbed onto the substrate particles, or may be prepared by methods known per se in the presence of the substrate particles, such as emulsion polymerisation. It is however preferably prepared directly on the plateletlike substrate particles, starting for example from monomers. With the latter new method, a much more regular coating is obtained and the number of loose particles is increased, giving surprising better coloristics.
The nitrogen doped carbon coating has for example a thickness of from 1 nm to 1 xcexcm, preferably of from 1 nm to 300 nm. Further preferences for particular coating compounds are given below.
Above the nitrogen doped carbon coating, the instant effect pigments may optionally also be coated with an outer coating, which may consist of one or more layers of various materials according to the function to be performed. For example, the outer coating may consist of a transparent or selectively absorbing dielectric material of any kind, the specific electrical resistance of which according to the customary definition is at least 1010xcexa9xc2x7cm.
Where appropriate, the outer coating preferably consists of a metal oxide, oxide hydrate or metal fluoride, for example of TiO2, ZrO2, SiO, SiO2, SnO2, GeO2, ZnO, Al2O3, V2O5, Fe2O3, Cr2O3, MgO, MgF2, CuO or PbTiO3, or a mixture thereof. Special preference is given to those metal oxides or oxide hydrates which are neither dissolved nor etched by the solvents used in many applications. Expediently, the outer coating should not impair the colorative properties of the coating system according to the invention located beneath it, but retain them as far as possible or even improve them. The person skilled in the art will know which material is suitable for which function, and which thicknesses are adequate.
The outer coating may protect the underlying coatings from chemical or mechanical influences. In this case, its refractive index is preferably as similar as possible to that of the external medium in which the pigment is intended to be embedded. Particularly preferred, the outer coating has a refractive index of from 1.33 to 1.71, although materials having high refractive indices may also be used. The thickness of a protective outer coating is most adequately no greater than 200 nm, preferably no greater than 100 nm, especially no greater than 50 nm.
The outer coating may, however, also reflect part of the incident light, or refract the incident light and the light reflected by the core, generating interference effects. In this case, its refractive index is preferably as high as possible, for example above 2.0. The thickness of a reflective outer coating is most adequately from 100 to 400 nm.
Of course, the outer coating may also consist of multiple layers, for example such as described above for the intermediate coating. When the outer coating consists of more than one layer, then it is preferably composed of alternate layers of a dielectric material and an instant nitrogen doped carbon coating or a semitransparent metal.